Sapflow+: a four-needle heat-pulse sap flow sensor enabling nonempirical sap flux density and water content measurements

? To our knowledge, to date, no nonempirical method exists to measure reverse, low or high sap flux density. Moreover, existing sap flow methods require destructive wood core measurements to determine sapwood water content, necessary to convert heat velocity to sap flux density, not only damaging the tree, but also neglecting seasonal variability in sapwood water content. ? Here, we present a nonempirical heat-pulse-based method and coupled sensor which measure temperature changes around a linear heater in both axial and tangential directions after application of a heat pulse. By fitting the correct heat conduction-convection equation to the measured temperature profiles, the heat velocity and water content of the sapwood can be determined. ? An identifiability analysis and validation tests on artificial and real stem segments of European beech (Fagus sylvatica L.) confirm the applicability of the method, leading to accurate determinations of heat velocity, water content and hence sap flux density. ? The proposed method enables sap flux density measurements to be made across the entire natural occurring sap flux density range of woody plants. Moreover, the water content during low flows can be determined accurately, enabling a correct conversion from heat velocity to sap flux density without destructive core measurements.     

Combining sap flow and trunk diameter measurements to assess water needs in mature olive orchards

Sap flux (Q) and trunk diameter variation (TDV) are among the most useful plant-based measurements to detect water stress and to evaluate plant water consumption. The usefulness of both methods decreases, however, when applied to species that, like olive, have an outstanding tolerance to drought and a remarkable capacity to take up water from drying soils. Evidence shows that this problem is greater in old, big trees with heavy fruit load. Our hypothesis is that the analysis of simultaneous measurements of Q and TDV made in the same trees is more useful for assessing irrigation needs in old olive orchards than the use of any of these two methods alone. To test our hypothesis, we analysed relations between Q, TDV, midday stem water potential (Ψ_stem), relative extractable water and atmospheric demand in an olive orchard of 38-year-old ‘Manzanilla’ trees with heavy fruit load. Measurements were made during one irrigation season (May-October), in fully irrigated trees (FI, 107% of the crop evapotranspiration, ET_c, supplied by irrigation), and in trees under two levels of deficit irrigation (DI60, 61% ET_c; DI30, 29% ET_c). Time courses of Q and TDV measured on days of contrasting weather and soil water conditions were analysed to evaluate the usefulness of both methods to assess the crop water status. We calculated the daily tree water consumption (E_p) from Q measurements. For both DI treatments we calculated a signal intensity by dividing daily E_p values of each DI tree by those of the FI tree (SI−Ep). We did the same with the maximum daily shrinkage (MDS) values (S_I−MDS). Neither SI−Ep nor S_I−MDS rendered useful information for assessing the crop water needs. On the contrary, the daily difference for maximum trunk diameter (MXTD) between each of the DI trees and the FI tree (D_MXTD) clearly indicated the onset and severity of water stress. A similar analysis with the E_p values, from which DEp values were derived, showed the effect of water stress on the water consumption of the trees. We concluded that the simultaneous use of D_MXTD and DEp values provides more detailed information to assess water needs in mature olive orchards than the use of Q or TDV records alone.     

Linking xylem diameter variations with sap flow measurements

Measurements of variation in the diameter of tree stems provide a rapid response, high resolution tool for detecting changes in water tension inside the xylem. Water movement inside the xylem is caused by changes in the water tension and theoretically, the sap flow rate should be directly proportional to the water tension gradient and, therefore, also linearly linked to the xylem diameter variations. The coefficient of proportionality describes the water conductivity and elasticity of the conducting tissue. Xylem diameter variation measurements could thus provide an alternative approach for estimating sap flow rates, but currently we lack means for calibration. On the other hand, xylem diameter variation measurements could also be used as a tool for studying xylem structure and function. If we knew both the water tension in the xylem and the sap flow rate, xylem conductivity and/or elasticity could be calculated from the slope of their relationship. In this study we measured diurnal xylem diameter variation simultaneously with sap flow rates (Granier-type thermal method) in six deciduous species (Acer rubrum L., Alnus glutinosa Miller, Betula lenta L., Fagus Sylvatica L. Quercus rubra L., and Tilia vulgaris L.) for 7–91 day periods during summers 2003, 2005 and 2006 and analyzed the relationship between these two measurements. We found that in all species xylem diameter variations and sap flow rate were linearly related in daily scale (daily average R ² = 0.61–0.87) but there was a significant variation in the daily slopes of the linear regressions. The largest variance in the slopes, however, was found between species, which is encouraging for finding a species specific calibration method for measuring sap flow rates using xylem diameter variations. At a daily timescale, xylem diameter variation and sap flow rate were related to each other via a hysteresis loop. The slopes during the morning and afternoon did not differ statistically significantly from each other, indicating no overall change in the conductivity. Because of the variance in the daily slopes, we tested three different data averaging methods to obtain calibration coefficients. The performance of the averaging methods depended on the source of variance in the data set and none of them performed best for all species. The best estimates of instantaneous sap flow rates were also given by different averaging methods than the best estimates of total daily water use. Using the linear relationship of sap flow rate and xylem diameter variations we calculated the conductance and specific conductivity of the soil–xylem–atmosphere water pathway. The conductance were of the order of magnitude 10⁻⁵ kg s⁻¹ MPa⁻¹ for all species, which compares well with measured water fluxes from broadleaved forests. Interestingly, because of the large sap wood area the conductance of Betula was approximately 10 times larger than in other species.     

Using branch and basal trunk sap flow measurements to estimate whole-plant water capacitance: a caution

Thermometric sap flow sensors are widely used to measure water flow in roots, stems and branches of plants. Comparison of the timing of flow in branches and stems has been used to estimate water capacitance of large trees. We review studies of sap flow in branches and present our own data to show that there is wide variation in the patterns and timing of sap flow of branches in different parts of the crown, owing to the course of daily solar illuminance. In contiguous forest, east-facing and upper branches are illuminated earlier than west-facing and lower branches and most capacitance studies do not include adequate information about branch sampling regimes relative to the overall pattern of crown illuminance, raising questions about the accuracy of capacitance estimates. Measuring only upper branches and normalising these results to represent the entire crown is dangerous because flows at the stem base likely peak in response to maximum crown illuminance (and transpiration) and this will differ compared to the timing of peak flows in upper branches. We suggest that the magnitude of flow lags between branches and stems needs further study, with careful attention to branch position and method application before a robust understanding of capacitance, particularly in woody tissues of large trees, can be formed. We did not detect flow lags in the world’s tallest and largest tree species Sequoia sempervirens and Sequoiadendron giganteum, despite measurement along large pathlengths (∼57 and 85 m), which raises questions as to why large flow lags are often recorded for much smaller species. One conspicuous possibility is the different methods used among studies. Constant-heating methods such as the thermal dissipation probe (and also heat balance methods) include heat capacitance behaviour due to warming of wood tissues, which delays the response of the sensors to changing sap flow conditions. We argue that methods with intrinsic heat-capacitance present dangers when trying to measure water-capacitance in trees. In this respect heat pulse methods hold an advantage.     

Estimation of whole-plant transpiration of bananas using sap flow measurements

Banana, one of the largest rhizomatous herbs in the world, is the fourth most important global food crop. It has a high water requirement, but the whole-plant water use in the field has not been determined satisfactorily. In this study, whole-plant water use in potted and field-grown banana plants (Musa 'Cavendish' cv. Williams) was successfully determined using a xylem sap flow method. This was achieved using Granier sensor probes implanted into the central cylinder of the banana corm. The whole-plant water use in field-grown bananas was 9-10 l plant(-1) d(-1). The values of daily total sap flow in potted plants correlated closely with gravimetric measurements (r(2)=0.92) and with changes in soil water status (r(2)=0.77). In well-watered, mature, field-grown plants, hourly sap flow also closely correlated with changes in solar radiation, vapour pressure deficit and evapotranspiration. The study indicates that sap flow measurement is a sensitive and accurate method for determining whole-plant water use in bananas under potted as well as field conditions.     

Gas-exchange and sap flow measurements of Salix viminalis trees in short-rotation forest

Simultaneous field measurements of transpiration and sap flow were performed on short-rotation Salix viminalis trees ranging in diameter from 1.5 to 3.5 cm (2-year-old shoots on 8-year-old stumps). Transpiration was measured using an open-top ventilated chamber enclosing the whole foliage of a tree. Sap flow was measured using a tree-trunk heat balance (THB) technique with a constant temperature difference and variable heat input. Both the instantaneous and daily values of water flux measured by the two absolute techniques agreed well with a difference of up to about 5%. In July, the hourly transpiration reached a maximum of about 0.2 kg m^–2 (leaf area) or 0.45 kg tree^–1, whereas maximum daily integrals reached 4 kg tree^–1. The response of sap flow rate to abrupt flux change when inducing emboli by cutting-off the stem was very rapid: the registered signal dropped by 85% within 10 min for a specimen with a projected leaf area of 2 m². For S. viminalis trees, transpiration was linearly correlated with stem cross-sectional area and with leaf area.     

基于树干液流及涡动相关技术的葡萄冠层蒸腾及蒸散发特征研究

针对西北干旱区绿洲经济作物葡萄树冠层蒸腾及蒸散发特征的相关问题,在甘肃省敦煌市南湖绿洲开展无核白葡萄树液流速率及蒸散发观测试验,采用基于热平衡原理的包裹式茎流计,详细分析了典型生长季7—9月份葡萄树蒸腾耗水规律,使用"单位叶面积上的平均液流速率SF×叶面积指数LAI"的方法,实现了从单株到林分冠层蒸腾的尺度扩展,并通过与涡动相关技术所测蒸散发数据对比,详细研究了葡萄地冠层蒸腾及蒸散发规律。结果表明:典型生长季中葡萄树液流速率日变化为单峰型曲线,日均耗水量从2.76 kg到10 kg不等,胸径越大的葡萄树日均耗水量越大;冠层蒸腾及蒸散发日变化曲线亦为单峰型,白天8:00—12:00与17:00—20:00期间,葡萄冠层蒸腾与蒸散发曲线均比较吻合,该时间段葡萄地蒸散发绝大部分来源于葡萄冠层蒸腾,而12:00—17:00之间由于午后太阳辐射强烈土壤蒸发量增加,葡萄蒸散发大于冠层蒸腾;典型生长季3个月中,葡萄冠层蒸腾量的变化范围在1.88—8.12 mm/d之间,日均冠层蒸腾量为6.12 mm/d,蒸散发在1.74 mm/d至10.78 mm/d之间,日均蒸散发量为7.13 mm/d;日均土壤蒸发量约为1.01 mm/d,只占总蒸散发量的14.2%,日均冠层蒸腾占日均蒸散发的比重达到85.8%,说明该生长阶段冠层蒸散发以作物蒸腾为主。     

Estimation of hydraulic conductance within field-grown apricot using sap flow measurements

Using the heat pulse and other techniques, the hydraulic architecture of apricot trees was mapped out. The flows (overall flow, flow across the four main branches) and forces (water potential differences between xylem and leaves) measured allowed us to quantify hydraulic conductance of branches and of the root/soil resistance. The experiment was carried out in a commercial orchard of 11-year-old apricot trees (Prunus armeniaca L., cv. Búlida, on Real Fino apricot rootstock) during 1 week (October 27–November 3, 1998). Three representative trees with a cylindrical trunk divided into four main branches of different sizes, orientation and local microclimate were chosen for the experiment. Sap flow was measured throughout the experimental period. Twelve sets of heat-pulse probes were used, one for each main branch. The diurnal course of the environmental conditions, the fraction of the area irradiated and leaf water relations were also considered in each main branch. The relationships between leaf water potential, xylem water potential and transpiration were established for different branches and also for the total plant. Using the slopes of these regressions, total plant conductance, the hydraulic conductance of the stem and root pathway, the hydraulic conductance of the canopy and the hydraulic conductance of each branch were estimated. Our findings show that the root conductance and the canopy hydraulic conductance are similar in magnitude. Leaf hydraulic conductance per leaf area unit was similar for each of the four branch orientations, indicating that, while the light microclimate has a dominant influence on transpiration, in this case it had little effect on the hydraulic properties of the canopy.     

基于树干液流和土壤-叶片水势梯度分析荷木干湿季整树水分利用特征

运用Granier热消散探针连续监测荷木的树干液流,于2009年的湿季(8月)和干季(11月)选择天气晴朗的3d测定叶片水势,同步连续监测林冠上方光合有效辐射、土壤含水量、气温和空气相对湿度.结果表明:干湿季下荷木树干液流存在显著差异,此外,土壤水势和液流有较好的相关性,且干季时的相关性更好;荷木的叶面积/边材面积比值平均为(0.416±0.033)m2·cm-2,并与树高呈指数函数下降关系;随着11月土壤水势下降,荷木的整树水力导度和午间叶片水势也有所下降,但不明显;对叶片水势和整树蒸腾进行回归分析,二者之间呈二次多项式关系(P＜0.01),叶片水势并非无限制下降;结果还表明,大气水汽压亏缺(D)和叶片水势呈负相关,这是否空气温度和相对湿度或共同作用影响叶片水势,需要进一步研究.     

Effect of stem water content on sap flow from dormant maple and butternut stems: induction of sap flow in butternut

Sap flow from excised maple stems collected over the winter (1986/87) was correlated with stem water content. Stem water content was high in the fall (>0.80) and decreased rapidly during 2 weeks of continuous freezing temperatures in late winter (<0.60). Exudation of sap from stem segments subjected to freeze/thaw cycles was small (<10 mL/kg) in the fall, but substantial exudation (45-50 mL/kg) occurred following the decline in water content. These observations are consistent with Milburn's and O'Malley's models (J.A. Milburn, P.E.R. O'Malley [1984] Can J Bot 62: 2101-2106; P.E.R. O'Malley, J.A. Milburn [1983] Can J Bot 61:3100-3106) of sap absorption into gas-filled fibers during freezing. Exudation volume was increased 200 to 300% in maple stems originally at high water content (>0.80) after perfusion with sucrose and dehydration at −12°C. Sap flow was also induced in butternut stem segments after the same treatment. Thus, sap flow may not be unique to maples. Sap flow could not be increased in stem segments dehydrated at 4°C. Migration of water molecules from small ice crystals in fibers to larger crystals in vessels while stems were frozen may account for increase exudation after dehydration at −12°C. This would result in preferential dehydration of fibers and a distribution of gas and sap favorable for stem-based sap flow.     

Relationships between sap flow and water potential in woody or perennial plants on islands of the Great Barrier Reef

Abstract. This paper describes studies on trees of Pisonia grandis , bushes of Argusia argentea , and the perennial herb Melanthera biflora , growing on One Tree Island, a coral cay of the Great Barrier Reef with 'soil' of coarse coral rubble. Water potential (Ψ_b, measured on small shoots with a pressure chamber), sap flow, stomatal conductance, vapour pressure deficit and photon flux density were monitored over day/night cycles. Sap flow and Ψ_b responded to changes in light and humidity. From these experiments good linear correlations were found between sap flow in a shoot and Ψ_b of similar adjacent shoots. The linearity suggests that the resistance to sap flow is constant as Ψ_b varies. The correlation, however, does not indicate a causal relationship between Ψ_b of an individual shoot on the plant and its sap flow. Ψ_b was only slightly different in shaded shoots from those in sunshine, although sap flow would be expected to differ between them. Enclosing shoots and so reducing their transpiration and sap flow to very low rates resulted in only small changes in Ψ_b of the enclosed shoots; Tb of such enclosed shoots should closely approximate that of the xylem at the point of shoot attachment. From these results it is suggested that the resistance to water flow in shoot and leaf xylem is small compared to the resistance further down the plant, in the root or at the root/soil interface. Shoot xylem water potential would be similar for all parts of the plant, and in such plants the water potential of shoots in the shade would be determined by the overall water use of the plant.     

基于茎流-蒸渗仪法的荒漠草原带人工灌丛群落蒸散特征

中国西北地区通过大量种植中间锦鸡儿(Caragana liouana)进行生态治理, 在荒漠草原带上形成人工灌丛景观, 改变了生态系统的结构和功能, 影响到地-气水汽循环过程, 研究该人工灌丛群落的蒸散特征, 对揭示其生态水文效应和指导地方生态治理实践具有重要意义。该文以宁夏盐池荒漠草原带上的人工灌丛群落为例, 利用茎流-蒸渗仪法测定了2018年5-8月的灌木蒸腾和丛下蒸散, 并分析了环境因子对人工灌丛群落蒸散的影响。结果表明: (1)茎流-蒸渗仪法所测的群落蒸散与水量平衡法、涡度相关法得到的群落蒸散有较好的一致性, 茎流-蒸渗仪法能适用于荒漠草原带人工灌丛群落蒸散及其组分结构的测定; (2)观测期内晴天的灌木蒸腾速率和丛下蒸散速率日变化趋势相近, 均为单峰曲线, 群落蒸散主要发生在日间, 但灌丛最大蒸腾速率的出现时间比丛下蒸散最大速率的出现时间晚1 h; (3) 5-8月间灌木累积蒸腾为83.6 mm, 日平均蒸腾量为0.7 mm·d^-1, 季节变化呈抛物线状; 同期丛下累积蒸散为182.5 mm, 日平均蒸散量为1.5 mm·d^-1; 丛下蒸散明显大于灌木蒸腾; (4)观测期间人工灌丛群落累积蒸散266.1 mm, 而同期的降水量为222.6 mm, 陆面水分收支处于亏缺状态; (5)净辐射是影响蒸散最主要、最直接的驱动因素, 且能够影响其他因子进而对人工灌丛群落蒸散产生作用。综上, 人工灌丛引发荒漠草原地带陆面水分收支亏缺的现象, 在生态恢复与重建中须引起注意。     

Visualisation of xylem sap flow direction in isolated fine lateral roots and estimation of the xylem sap osmotic potential

Xylem sap outflow from fine lateral roots (FLRs) isolated from hydroponically grown young maize (Zea mays L.) plants was visualized by local brightening of test solutions contrasted with purified Indian ink particles. Flow into the vessels was indicated by the adsorption of Evans Blue in their walls. The fraction of the FLRs able to exude xylem sap in a mineral medium with 30 mM mannitol decreased with increasing incubation time. This change was strongly retarded, when the FLRs were incubated in a medium containing glucose instead of mannitol. There was a broad range of variation of the osmotic potential of the test solutions (Ψ_so), wherein the fraction of the FLRs showing an initially reversed flow of the xylem sap varied between zero and unity. A median (M) of the osmotic potential of the xylem sap in FLRs (Ψ_sx) was estimated. It represents the value of Ψ_so that was lower than Ψ_sx in half of the roots of a sample before their transfer to the test solutions (Ψ_sxo). M was dependent on the osmotic potential of the medium used for growth or pre-incubation of the FLRs. Its value was not dependent on the molecular size of the osmolytes used to adjust Ψ_so, including dextran 8, which is excluded from cell walls. In all of the studied plants, M was lower than the osmotic potential of the xylem sap collected from the root before isolation of the FLRs. To explain this finding it is assumed that FLRs with Ψ_sxo > M had a higher hydraulic conductivity and a larger volume contributed to the exuded sap than those with Ψ_sx < M.     

马占相思夜间树干液流特征和水分补充现象的分析

研究马占相思树干的夜间水分补充现象有助于提高总蒸腾量、冠层气孔导度估算的准确度,同时可以深化对冠层蒸腾与树干液流所代表的日蒸腾之间时滞现象的认识。在广东鹤山马占相思林,采用热扩散探针法测定液流密度,同步测定主要的环境因子,从不同的时间尺度分析其树干夜间水分补充现象。结果表明:与白天相比较,夜间马占相思液流密度较小;夜间液流密度的变化幅度旱季比雨季大,树干夜间水分补充的主要时间段是前半夜;年内各个月份夜间水分补充量之间没有显著差异,它与环境因子之间相关关系不显著,但与胸径、树高、冠幅的回归曲线拟合较好;干季夜间补充的水分对总蒸腾量的贡献与白天蒸腾量、总的蒸腾量、水蒸气压亏缺(VPD)、光合有效辐射(PAR)以及大气温度(T)显著相关;湿季夜间补充的水分对总蒸腾量的贡献与白天蒸腾量、总蒸腾量显著相关。     

Characteristics of evapotranspiration in planted shrub communities in desert steppe zone based on sap flow and lysimeter methods

中国西北地区通过大量种植中间锦鸡儿(Caragana liouana)进行生态治理, 在荒漠草原带上形成人工灌丛景观, 改变了生态系统的结构和功能, 影响到地-气水汽循环过程, 研究该人工灌丛群落的蒸散特征, 对揭示其生态水文效应和指导地方生态治理实践具有重要意义。该文以宁夏盐池荒漠草原带上的人工灌丛群落为例, 利用茎流-蒸渗仪法测定了2018年5-8月的灌木蒸腾和丛下蒸散, 并分析了环境因子对人工灌丛群落蒸散的影响。结果表明: (1)茎流-蒸渗仪法所测的群落蒸散与水量平衡法、涡度相关法得到的群落蒸散有较好的一致性, 茎流-蒸渗仪法能适用于荒漠草原带人工灌丛群落蒸散及其组分结构的测定; (2)观测期内晴天的灌木蒸腾速率和丛下蒸散速率日变化趋势相近, 均为单峰曲线, 群落蒸散主要发生在日间, 但灌丛最大蒸腾速率的出现时间比丛下蒸散最大速率的出现时间晚1 h; (3) 5-8月间灌木累积蒸腾为83.6 mm, 日平均蒸腾量为0.7 mm·d^-1, 季节变化呈抛物线状; 同期丛下累积蒸散为182.5 mm, 日平均蒸散量为1.5 mm·d^-1; 丛下蒸散明显大于灌木蒸腾; (4)观测期间人工灌丛群落累积蒸散266.1 mm, 而同期的降水量为222.6 mm, 陆面水分收支处于亏缺状态; (5)净辐射是影响蒸散最主要、最直接的驱动因素, 且能够影响其他因子进而对人工灌丛群落蒸散产生作用。综上, 人工灌丛引发荒漠草原地带陆面水分收支亏缺的现象, 在生态恢复与重建中须引起注意。     

Tree water dynamics in a drying and warming world

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (F_D), stomatal conductance (G_s), hydraulic conductivity (K_L) and xylem anatomy in piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower G_s and F_D were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of G_s and F_D to ambient levels only for piñon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced F_D, G_s and K_L. Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in G_s, F_D and K_L took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi‐arid regions.     

Using branch and basal trunk sap flow measurements to estimate whole-plant water capacitance: comment on Burgess and Dawson (2008)

Plant and Soil - Sap flow sensors are uniquely able to continuously monitor whole tree physiology. Recently, Burgess and Dawson (Burgess SSO, Dawson TE, Plant Soil 305:5–13, 2008) urged...     

Compensation heat-pulse measurements of sap flow for estimating transpiration in young lemon trees

Potted two-year-old lemon trees [Citrus limon (L.) Burm. f.], cv. Verna grafted on sour orange (C. aurantium L.) rootstock, growing in greenhouse, were subjected to drought for 33 d. Control plants were daily irrigated at field capacity. Values of sap flow (SF) were compared with transpiration (E) rates measured gravimetrically. The results underlined the robustness and high sensitivity of the compensation heat-pulse technique for estimating transpiration on a wide range of SF. Good direct correlations between E and SF rates on an instantaneous and daily basis were obtained in both treatments. On a daily basis, a common calibration curve can be used for both irrigation treatments. On an instantaneous basis, changes in SF were matches by similar changes in E in both treatments, although the relationships between these parameters presented different intercepts in each treatment. Sap flow rates were influenced by weather conditions in trees growing in non-limiting soil water conditions. This makes it possible to evaluate the significance of any sap flow measurement in relation to the reference value calculated for the vapour pressure deficit at the time the measurement was taken.This research was supported by Ministerio de Educacion y Ciencia (MEC), (CICYT/FEDER AGL2003-9387-C05-02 and AGL2004-0794-C03-02), and PETRI (PTR1995-0693-OP-02-01) grants to the authors. M.F. Ortuno was a recipient of a Program I3P research fellowship from CSIC.     

塔克拉玛干沙漠腹地多枝柽柳茎干液流及耗水量

2005年4—11月,利用植物茎流计研究了塔克拉玛干沙漠腹地沙漠公路防护林植物多枝柽柳的茎干液流特性和耗水量.结果表明:在极端干旱的沙漠腹地,土壤水分充足时,直径为3.5和2.0 cm的多枝柽柳在整个生长季的日平均耗水量分别为6.322和1.179 kg;多枝柽柳的茎干液流呈单峰曲线型,有明显的昼夜变化规律,茎干液流随环境因子变化而波动;在土壤水分充足的条件下,总辐射、风速、温度是影响茎干液流变化的主要环境因子,可以用总辐射和风速的线性回归模型预测茎干液流的变化.沙漠腹地多枝柽柳的蒸腾耗水量相对较高,是因为在较为充足的水分供应条件下,多枝柽柳通过大量的水分消耗来抵御干燥高温的沙漠环境.     

Evaluation of sap flow methods to determine water use by cultivated palms

Sustainable management of water resources allocated to palm crops requires reliable tools for measuring palm water use. Thermometric sap flow methods developed for woody dicot plants hold great potential for use in palms, but there have been few investigations to determine whether such methods can be used successfully in ‘woody’ monocots. Here, we evaluate two sap flow methods for measuring whole-plant water use by potted cocos palms (Syagrus romanzoffiana): the Heat Ratio Method (HRM) and the Compensation Heat Pulse Method (CHPM). Measurements of whole-plant water use from the HRM and the CHPM were compared to gravimetric measurements acquired from an electronic balance. Of the two methods, the HRM gave the most accurate results and it most precisely described patterns of transpiration with respect to environmental conditions and leaf morphology. Estimates of total daily water use from the HRM and the balance were highly correlated (R² = 0.92; P < 0.0001) and very near to a 1:1 relationship—an excellent result given the potential for error associated with each method. As expected, the CHPM was seriously limited at low flow rates, but it agreed well with the HRM at higher flow rates (heat pulse velocity >4 cm h⁻¹). Anatomical investigations revealed that vascular bundles in measured palm fronds were evenly distributed and the distance between bundles was comparable in scale to the diameter of probes of HRM sensors, and most likely an order of magnitude smaller than the probable zone of thermal influence for HRM measurements. This contention was supported by results of mathematical modeling suggesting that HRM heat pulse velocities in palm-like sapwood remain largely unaffected by increases in heterogeneity caused by larger xylem vessels and wider interstitial tissues. Although wounding models for heat pulse sap flow methods have not historically catered for the effects of discrete vascular bundles, they are sufficiently approximate given our empirical validations, and we conclude that palm sapwood is thermally homogenous enough for the HRM and the CHPM to be used without modification. Overall, we present strong evidence that heat pulse methods such as the HRM and the CHPM can be used successfully in woody monocots. Finally, we note that the CHPM is likely to perform better in cases of very high flow rates, while the HRM provides more comprehensive monitoring of the usual range of flow rates including at night, early morning and during water deficit.